Building stairs looks straightforward until you realize every riser must land within ⅜" of the others — and that total rise, the starting measurement, is the one number most DIYers get wrong. Miss it and every step in the staircase is off. Get any dimension outside the allowable range and the inspector fails the project.
What Stair Rise and Run Mean
Rise is the vertical height of a single step — the distance from the top of one tread to the top of the next. Run(also called the “going”) is the horizontal depth of each tread, measured nosing to nosing. Total rise is the full vertical distance the staircase spans: finished floor to finished floor (or finished deck surface).
One distinction that trips up beginners: stairs always have one more riser than tread. A staircase with 14 risers has 13 treads — the top landing counts as the 14th riser but not as a separate tread.
How to Calculate Stair Rise and Run
Follow this sequence for any staircase:
- Measure total rise in inches. This is the vertical distance from the finished floor below to the finished floor above — including any flooring material not yet installed.
- Find the number of risers. Divide total rise by 7 (a comfortable target rise) and round up to the nearest whole number. This gives the minimum riser count that keeps each step at or below 7 inches.
- Calculate unit rise.Divide total rise by the riser count. Verify it doesn't exceed 7¾" — the IRC maximum for residential stairs.
- Set tread depth.Each tread must be at least 10" of usable run (excluding any nosing overhang). Most builders use 10½" to 11" for comfort.
Worked example — 9-ft deck
Suppose your deck surface is 108 inches above grade. Here's the math:
Total rise: 108 in
108 ÷ 7 = 15.4 → round up to 16 risers
Unit rise = 108 ÷ 16 = 6.75 in ✓ (under 7¾" max)
Tread depth = 11 in ✓ (over 10" min)
Comfort check: 2R + T = 2(6.75) + 11 = 24.5 in ✓ (range: 24–25 in)For a different height — or to get stringer length, total run, and material counts — you can run the full stair calculation here for both deck and interior runs.
Building Code Limits for Rise and Run
The International Residential Code (IRC) sets the allowable range for residential stairways. Most US jurisdictions adopt the IRC, though some states and municipalities have stricter local amendments — always verify with your local building department.
| Dimension | IRC Limit | Notes |
|---|---|---|
| Max unit rise | 7¾ in | Vertical height per step |
| Min unit run | 10 in | Tread depth, excluding nosing overhang |
| Max variation | ⅜ in | Between any two adjacent risers — or any two adjacent treads |
| Min headroom | 6 ft 8 in | Measured vertically from nosing to ceiling above |
| Min stair width | 36 in | Clear width between handrails or walls |
The ⅜" variation rule is what catches most DIY stairs at inspection. Even if every riser is individually under 7¾", a mix of 6¾" and 7¼" steps will fail — the difference between any two adjacent risers can't exceed ⅜". The same limit applies to treads.
The 2R + T Comfort Formula
Building code sets the allowable range, but comfortable stairs live in a narrower band. Stair designers use a rule of thumb: two times the unit rise plus the tread depth (2R + T) should equal 24–25 inches. This approximates a natural walking stride projected onto a slope.
A 7" rise with an 11" tread gives 2(7) + 11 = 25 — at the upper edge, slightly steep. A 6.5" rise with an 11" tread gives 24 — at the lower edge, more leisurely. Code-legal stairs can still feel uncomfortably steep or too open if the rise and tread aren't balanced — this formula keeps them in proportion.
Common Mistakes That Cause Non-Code Stairs
- Measuring to subfloor, not finished floor.If you're adding ¾" hardwood upstairs, that ¾" reduces your total rise. The change propagates to every riser height — always measure to the finished surface before calculating.
- Making the bottom step shorter than the rest. When stringers are set and then flooring is added at the base, the first riser often ends up taller or shorter than the others. Account for the floor finish thickness at both the top and bottom before cutting stringers.
- Confusing treads and risers when ordering material.A 14-riser stair needs 13 tread boards — the top landing is the 14th riser and doesn't need a separate tread. Ordering 14 boards wastes one; ordering 12 leaves you short.
- Not measuring variation after installation.Wood swells and shrinks. Check final riser heights after the tread boards are installed, not just at the framing stage — small irregularities get amplified and can push you over the ⅜" variation limit.
Getting the math right before cutting stringers is the single biggest time-saver in stair construction. A staircase that fails the ⅜" variation check means tearing out installed treads — a mistake that a ten-minute calculation prevents.